When Is It Finished ?

If you have a deadline, you let the baby go. If it's for your own fun, you most likely come back to it again and again for varying reasons. I'm sure that when something like, say, Melodyne came out, many, many of us opened some old projects to see what miracles they could now perform.
 
You don't need to have a strict policy, do what you like! If it really bothers you, stop doing it. It may be as hard as quitting smoking, though.

I sort of have to get the song finished in a few weeks or it must have been a stupid song anyhow and I will abandon it. A good song will write itself, and with the tools we now have I don't have excuses anymore,, I even have my own drums! I often get them done in a few days. 
Unless I need other musicians involved then they take longer. 
The only songs I'm re doing are ones I recorded in the early 90's to multi track cassette. The performances where excellent, the sound quality drove me nuts.  
I'm just about finished my Acoustic original re visited and next is the Country Rock sort of songs I'll re-do. Last and best will be my favorite, Johnny's Blues! That will involve some other people including my old pals " The Hounds of Sound" horn section.